Weldability of marine grade AA 5052 aluminum alloy by underwater friction stir welding

Friction stir welding (FSW) is a solid-state joining process producing high-quality welds with lower residual stresses and improved mechanical properties. Underwater FSW is a variant of FSW process which controls heat conduction and dissipation along the weld line improving the joint properties. The feasibility of underwater friction stir welding of AA 5052 H32 aluminum alloy to improve the joint performance than normal friction stir welding is addressed in this paper. The effects of tool rotational speed and welding speed on ultimate tensile strength by underwater and normal friction stir welding were analyzed and compared. It was observed that the tensile strength of underwater welded joints was higher than normal FSW joints except at 500 rpm. Maximum tensile strength of 208.9 MPa was obtained by underwater friction stir welding at 700 rpm tool rotational speed and welding speed of 65 mm/min. The optimum process parameters for achieving maximum tensile strength by normal FSW were compared with underwater FSW. The result showed that the ultimate tensile strength obtained by underwater FSW was about 2% greater than that of the normal FSW process. The joints with maximum tensile strength during underwater and normal welding fractured at the retreating side of the welded joint. Microstructural examination revealed that heat-affected region was not found in underwater welding. Microhardness was decreased slightly towards the stir zone. Fractography observation revealed that the welded joints exhibiting higher joint efficiency failed under ductile mode.